In fiber optic communications, dense wavelength division multiplexing (DWDM) is a technique for multiplexing multiple optical carrier signals onto a single optical fiber. This form of frequency division multiplexing is commonly referred to as DWDM when applied to optical systems that employ a high level of multiplexing. The potential of optical fibers is more fully exploited when multiple beams of light at different frequencies (wavelengths) are transmitted on the same fiber. By using different wavelengths of laser light to carry different signals, capacity is multiplied. In a DWDM system, a multiplexer is used at the transmitter to join the signals together and a de-multiplexer is used at the receiver to split the signals apart.
An optical ring resonator is a device that is capable of both multiplexing and de-multiplexing, and it can function as an add-drop multiplexer on a fiber-optic communication bus. Optical ring resonators include a waveguide in a closed loop, coupled to one or more input/output (or bus) waveguides. When light of the appropriate wavelength is coupled from an input waveguide to the ring, constructive interference causes a buildup in intensity over multiple round-trips through the ring. The light is ultimately coupled to an output waveguide. Since only selected wavelengths resonate in the ring, the ring functions as a filter. A range of applications such as optical switching, electro-optical switching, wavelength conversion, and filtering have been demonstrated using optical ring resonators.
An optical ring resonator waveguide is a transmission line that transmits light via refractions of light off the boundary layer between the walls of the optical waveguide and cladding covering the walls of the optical waveguide. The angle of refraction is wavelength dependent, so some wavelengths will propagate through the waveguide, and other wavelengths will be attenuated, allowing control over which wavelengths can travel in the optical waveguide. Furthermore, only wavelengths that divide evenly into the path length of the ring resonator are trapped, or filtered, from the waveguide into the ring.
The waveguides may be simple ridges or a composition of multiple parallel ridges as observed in slotted waveguides. A ridge waveguide is a convex silicon waveguide formed on an insulative silicon oxide substrate. A slotted waveguide is a concave waveguide formed as a slotted air-gap structure between two high refraction index waveguides. The optical characteristics of the ridge-waveguide ring resonators or slotted-waveguide ring resonators may be effectively changed to control the transmissible wavelengths by adding a cladding layer to the ridges or slots whose refractive index changes with applied electrical or intense optical fields.
Existing technologies for optical ring resonators: (1) switch too slowly (e.g., no faster than the microsecond, and typically millisecond range); (2) only switch over a very limited wavelength range, namely, a single full width at half maximum (FWHM); (3) are not readily compatible with silicon processing; (4) do not readily scale due to their large size which limits the number of ring resonators that can fit on a single silicon chip (owing their large size to their traditional fabrication process/materials); (5) are limited to configurable DWDM circuits; (6) do not readily extend over a 20 nanometer free spectral range required for wavelength switching across the C-band; and (7) do not operate at practical voltages since electro-optic coefficients are insufficient.